Progress in the semiconductor industry has been following the trend of Moore's Law that was proposed in 1965 by then Intel's cofounder Gordon Moore. This trend requires that the capability of integrated circuits (IC) or, in general, semiconductor chips double every 18 months. Thus, the number of transistors on a central processing unit (CPU) in 2002 may approach 100 million. As a result of this densification of circuitry, line-width in 2002 narrowed to 0.18 micrometer and more advanced chips are using wires as thin as 0.13 micrometer. With this trend continuing, it is projected that the seemingly impermeable “Point One” barrier, of 0.1 micrometer, will be attained and surpassed in the next few years.
Along with such advances comes various design challenges. One of the often overlooked challenges is that of heat dissipation. Most often, this phase of design is neglected or added as a last minute design before the components are produced. According to the second law of thermodynamics, the more work that is performed in a closed system, the higher entropy it will attain. With the increasing power of a CPU, the larger flow of electrons produces a greater amount of heat. Therefore, in order to prevent the circuitry from shorting or burning out, the heat resulting from the increase in entropy must be removed. The state-of-the-art CPU in 2002 has a power of about 60 watts (W). CPUs made with 0.13 micrometer technology will exceed 100 watts. Current methods of heat dissipation, such as by using metal (e.g., Al or Cu) fin radiators, and water evaporation heat pipes, will be inadequate to sufficiently cool future generations of CPUs.
Recently, ceramic heat spreaders (e.g., AlN) and metal matrix composite heat spreaders (e.g., SiC/Al) have been used to cope with the increasing amounts of heat generation. However, such materials have a thermal conductivity that is no greater than that of Cu, hence, their ability to dissipate heat from semiconductor chips is limited.
A typical semiconductor chip contains closely packed metal conductor (e.g., Al, Cu) and ceramic insulators (e.g., oxide, nitride). The thermal expansion of metal is typically 5–10 times that of ceramics. When the chip is heated to above 60° C., the mismatch of thermal expansions between metal and ceramics can create microcracks. The repeated cycling of temperature tends to aggravate the damage of the chip. As a result, the performance of the semiconductor will deteriorate. Moreover, when temperatures reach more than 90° C., the semiconductor portion of the chip may become a conductor so the function of the chip is lost. In addition, the circuitry may be damaged and the semiconductor is no longer usable (i.e. becomes “burned out”). Thus, in order to maintain the performance of the semiconductor, its temperature must be kept below a threshold level (e.g., 90° C.).
A conventional method of heat dissipation is to contact the semiconductor with a metal heat sink. A typical heat sink is made of aluminum that contains radiating fins. These fins are attached to a fan. Heat from the chip will flow to the aluminum base and will be transmitted to the radiating fins and carried away by the circulated air via convection. Heat sinks are therefore often designed to have a high heat capacity to act as a reservoir to remove heat from the heat source.
The above heat dissipation methods are only effective if the power of the CPU is less than about 60 W. For CPUs with higher power, more effective means must be sought to keep the hot spot of the chip below the temperature threshold.
Alternatively, a heat pipe may be connected between the heat sink and a radiator that is located in a separated location. The heat pipe contains water vapor that is sealed in a vacuum tube. The moisture will be vaporized at the heat sink and condensed at the radiator. The condensed water will flow back to the heat sink by the wick action of a porous medium (e.g., copper powder). Hence, the heat of a semiconductor chip is carried away by evaporating water and removed at the radiator by condensing water.
Although heat pipes and heat plates may remove heat very efficiently, the complex vacuum chambers and sophisticated capillary systems prevent designs small enough to dissipate heat directly from a semiconductor component. As a result, these methods are generally limited to transferring heat from a larger heat source, e.g., a heat sink. Thus, removing heat via conduction from an electronic component is a continuing area of research in the industry.
One promising alternative that has been explored for use in heat sinks is diamond-containing materials. Diamond can carry away heat much faster than any other material. The thermal conductivity of diamond at room temperature (about 2000 W/mK) is much higher than either copper (about 400 W/mK) or aluminum (250 W/mK), the two fastest metal heat conductors commonly used. Moreover, the thermal capacity of diamond (1.5 J/cm3) is much lower than copper (17 J/cm3) or aluminum (24 J/cm3). The ability for diamond to carry away heat without storing it makes diamond an ideal heat spreader. In contrast to heat sinks, a heat spreader acts to quickly conduct heat away from the heat source without storing it. Table 1 shows various thermal properties of several materials as compared to diamond (values provided at 300 K).
TABLE 1ThermalThermalConductivityHeat CapacityExpansionMaterial(W/mK)(J/cm3 K)(1/K)Copper4013.441.64E−5Aluminum2372.44 2.4E−5Molybdenum1382.574.75E−5Gold3172.491.43E−5Silver4292.471.87E−5Silicon1481.662.58E−6Diamond (IIa)2,3001.78 1.4E−6
In addition, the thermal expansion coefficient of diamond is one of the lowest of all materials. The low thermal expansion of diamond makes joining it with low thermally expanding silicon semiconductor much easier. Hence, the stress at the joining interface can be minimized. The result is a stable bond between diamond and silicon that does not delaminate under the repeated heating cycles.
In recent years diamond heat spreaders have been used to dissipate heat from high power laser diodes, such as that used to boost the light energy in optical fibers. However, large area diamonds are very expensive, hence, diamond has not been commercially used to spread the heat generated by CPUs. In order for diamond to be used as a heat spreader, its surface must be polished so it can make an intimate contact with the semiconductor chip. Moreover, its surface may be metallized (e.g., by Ti/Pt/Au) to allow attachment to a conventional metal heat sink by brazing.
Many current diamond heat spreaders are made of diamond films formed by chemical vapor deposition (CVD). The raw CVD diamond films are now sold at over $10/cm2, and this price may doubled when it is polished and metallized. This high price would prohibit diamond heat spreaders from being widely used except in those applications (e.g., high power laser diodes) where only a small area is required or no effective alternative heat spreaders are available. In addition to being expensive, CVD diamond films can only be grown at very slow rates (e.g., a few micrometers per hour); hence, these films seldom exceed a thickness of 1 mm (typically 0.3–0.5 mm). However, if the heating area of the chip is large (e.g., a CPU), it is preferable to have a thicker (e.g., 3 mm) heat spreader.
As such, cost effective devices that are capable of effectively conducting heat away from a heat source, continue to be sought through ongoing research and development efforts.